Transparent electrode solar cell

ABSTRACT

A transparent electrode with a transparent substrate and a composite layer disposed thereon, wherein the composite layer includes a graphene layer and a plurality of nanoparticles, wherein the nanoparticles are embedded in the graphene layer and extend through a thickness of the graphene layer, and wherein the plurality of nanoparticles are in direct contact with the transparent substrate and a gap is present between the graphene layer and the transparent substrate.

BACKGROUND OF THE INVENTION Technical Field

The present invention relates to a transparent electrode with atransparent substrate and a composite layer having a graphene layer anda plurality of nanoparticles embedded therein, wherein a gap is presentbetween the graphene layer and the transparent substrate.

Description of the Related Art

The “background” description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it is described in thisbackground section, as well as aspects of the description which may nototherwise qualify as prior art at the time of filing, are neitherexpressly or impliedly admitted as prior art against the presentinvention.

Graphene has some fascinating physical properties that are currently notfully utilized. For example, graphene has very high electronic mobility.Several studies reported the average electronic mobility of graphene tobe at least 15,000 cm²/(V·s), with a theoretical potential limit ofaround 200,000 cm²/(V·s) for a free-standing monolayer graphene. Thetheoretical upper bound is limited by the scattering of graphene'sacoustic photons. In a free-standing monolayer graphene, electrons actlike photons and are able to travel sub-micrometer distances withoutscattering. However, the electron mobility of graphene is substantiallylimited due to the presence of a substrate that carries the graphene.With silicon dioxide as the substrate, for example, the electronmobility of a graphene is substantially reduced to a value of no morethan 40,000 cm²/(V·s).

In view of the forgoing, one objective of the present disclosure is toprovide a transparent electrode with a transparent substrate and acomposite layer disposed thereon, wherein the composite layer includes agraphene layer and a plurality of nanoparticles, wherein thenanoparticles are embedded in the graphene layer and extend through athickness of the graphene layer, and wherein the plurality ofnanoparticles are in direct contact with the transparent substrate and agap is present between the graphene layer and the transparent substrate.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect, the present disclosure relates to atransparent electrode including i) a transparent substrate, ii) acomposite layer disposed on the transparent substrate, wherein thecomposite layer includes a graphene layer and a plurality ofnanoparticles, wherein the nanoparticles are embedded in the graphenelayer and extend through a thickness of the graphene layer, and whereinat least a portion of the plurality of nanoparticles are in directcontact with the transparent substrate and a gap is present between thegraphene layer and the transparent substrate

In one embodiment, a ratio of the thickness of the graphene layer to anaverage diameter of the plurality of nanoparticles is 1:1.5 to 1:10.

In one embodiment, the nanoparticles are at least one selected from thegroup consisting of a boron nitride nanotube, a boron nitride fullerene,a titanium oxide nanotube, and a titanium oxide fullerene.

In one embodiment, the nanoparticles are boron nitride nanotubes with anaverage diameter of 3-10 nm.

In one embodiment, the nanoparticles have a morphology selected from thegroup consisting of a tubular morphology, a spherical morphology, anelliptical morphology, and an oblong morphology.

In one embodiment, the gap defines a distance between the graphene layerand the transparent substrate, wherein an average size of the gap is0.1-10 nm.

In one embodiment, the composite layer further includes a p-type dopant,which is dispersed in the graphene layer.

In one embodiment, the p-type dopant is diborane.

In one embodiment, the composite layer further includes an n-typedopant, which is dispersed in the graphene layer.

In one embodiment, the n-type dopant is ammonia.

In one embodiment, the composite layer further includes quantum dots.

In one embodiment, the transparent substrate is at least one selectedfrom the group consisting of quartz, glass, polyethylene terephthalate,polybutylene terephthalate, polyacrylate, polymethacrylate,polymethylmethacrylate, polydimethylsiloxane, polyethylene,polypropylene, polyvinyl chloride, polyethernitrile, polyethersulfone,polystyrene, polycarbonate, styrene acrylonitrile, styrene methylmethacrylate, and methyl metacrylate butadiene styrene.

In one embodiment, the composite layer has an electron mobility of atleast 30,000 cm²/(V·s) at a temperature of 20-30° C.

In one embodiment, the composite layer has a light transmittance of atleast 95% when exposed to a light radiation with a wavelength of 300 to1,000 nm.

According to a second aspect, the present disclosure relates to a solarcell including i) the transparent electrode, ii) a cathode, iii) anelectrolyte arranged between the transparent electrode and the cathode,wherein the transparent electrode is in electric communication with thecathode through the electrolyte.

According to a third aspect, the present disclosure relates to a methodof fabricating the transparent electrode involving i) depositing theplurality of nanoparticles on a surface of a CVD substrate, ii) heatingthe CVD substrate and said nanoparticles to a temperature of at least900° C. at a sub-atmospheric pressure, iii) contacting a gaseous mixturecomprising methane and hydrogen with said nanoparticles and the CVDsubstrate, wherein hydrogen catalyzes a reaction between methane and theCVD substrate to deposit carbon atoms on the surface of the CVDsubstrate thereby forming the composite layer, wherein the nanoparticlesare embedded in the graphene layer and extend through a thickness of thegraphene layer, iv) cooling the CVD substrate and the composite layer,v) disposing the transparent substrate on the composite layer, vi)removing the CVD substrate with an etchant thereby fabricating thetransparent electrode, wherein said nanoparticles are in direct contactwith the transparent substrate and a gap is present between the graphenelayer and the transparent substrate.

In one embodiment, the transparent substrate is a polymeric material,wherein disposing the transparent substrate on the composite layer iscarried out by coating the composite layer with the polymeric material.

In one embodiment, the CVD substrate is a copper substrate, wherein theetchant is ferric chloride.

In one embodiment, the polymeric material is polydimethylsiloxane and/orpolymethylmethacrylate, wherein the organic solvent is acetone.

In one embodiment, the gaseous mixture further includes diborane,wherein the composite layer is a p-type semiconductor.

In one embodiment, the gaseous mixture further includes ammonia, whereinthe composite layer is an n-type semiconductor.

The foregoing paragraphs have been provided by way of generalintroduction, and are not intended to limit the scope of the followingclaims. The described embodiments, together with further advantages,will be best understood by reference to the following detaileddescription taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1A schematically illustrates a side-view of a CVD substrate duringdepositing nanoparticles.

FIG. 1B schematically illustrates a top-view of the CVD substrate afterdepositing the nanoparticles thereon.

FIG. 1C schematically illustrates a side-view of the CVD substrate afterdepositing the nanoparticles thereon.

FIG. 1D schematically illustrates a top-view of the CVD substrate duringdepositing graphene.

FIG. 1E schematically illustrates a side-view of the CVD substrateduring depositing graphene.

FIG. 1F schematically illustrates a top-view of the CVD substrate afterdepositing graphene thereon.

FIG. 1G schematically illustrates a side-view of the CVD substrate afterdepositing graphene thereon.

FIG. 1H schematically illustrates a top-view of the CVD substrate afterdepositing graphene, wherein the graphene is deposited in the presenceof a dopant.

FIG. 1I schematically illustrates a side-view of the CVD substrate afterdepositing graphene, wherein the graphene is deposited in the presenceof a dopant.

FIG. 1J schematically illustrates processing steps of fabricating thetransparent electrode: (1) disposing the transparent substrate on thecomposite layer, (2) removing the CVD substrate, (3) optionally removingthe transparent substrate, and (4) optionally placing the compositelayer on another substrate.

FIG. 2A schematically illustrates a solar cell, wherein the transparentsubstrate is the working electrode of the solar cell.

FIG. 2B schematically illustrates a magnified view of the workingelectrode of the solar cell.

FIG. 2C schematically illustrates a dye-sensitized solar cell, whereinthe transparent substrate is the working electrode of the dye-sensitizedsolar cell.

FIG. 2D schematically illustrates a magnified view of the workingelectrode of the dye-sensitized solar cell.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Embodiments of the present disclosure will now be described more fullyhereinafter with reference to the accompanying drawings, in which some,but not all embodiments of the disclosure are shown.

The present disclosure will be better understood with reference to thefollowing definitions. As used herein, the words “a” and “an” and thelike carry the meaning of “one or more.” Within the description of thisdisclosure, where a numerical limit or range is stated, the endpointsare included unless stated otherwise. Also, all values and subrangeswithin a numerical limit or range are specifically included as ifexplicitly written out.

Referring to FIGS. 1A-1J, according to a first aspect, the presentdisclosure relates to a transparent electrode 114. The term “transparentelectrode” as used in this disclosure refers to an electrode, whichpreferably has a light transmittance of at least 50%, preferably atleast 60%, preferably 65-95%, preferably 75-90% when exposed to a lightradiation with a wavelength of 300-1,000 nm, preferably 350-900 nm,preferably 400-800 nm. In terms of the present disclosure, transmittanceof a material is an ability of the material to transmit radiant energytherethrough; therefore, the “light transmittance” is a percentage ofradiant energy that is transmitted through a material which is not lostdue to absorption, scattering, reflection, etc.

The transparent electrode 114 includes at least a transparent substrate110 and a composite layer 112, as shown in FIG. 1J. Each component ofthe transparent electrode is fully described hereinafter.

The term “transparent substrate” refers to a transparent material thatis used as a support for the composite layer 112. In view of that, thetransparent substrate 110 may be any one of glass, quartz, a transparentpolymeric material, etc. In one embodiment, the transparent substrate110 is planar (in the form of a plane). In certain embodiments, athickness of the transparent substrate 110 may be within the range of 5to 500 μm, preferably 10 to 400 μm, more preferably 50 to 300 μm.Alternatively, the transparent substrate 110 may have a rectangularshape with a thickness of 1 to 10 mm, preferably 2 to 8 mm, preferably 3to 5 mm. In certain embodiments, the transparent substrate 110 may havea cylindrical shape or a spherical shape. The transparent substrate 110may be flexible or rigid depending on the application of the transparentelectrode 114.

In some embodiments, the transparent substrate 110 has a lighttransmittance of at least 50%, preferably at least 60%, preferably65-95%, preferably 75-90% when exposed to a light radiation with awavelength of 300-1,000 nm, preferably 350-900 nm, preferably 400-800nm.

In a preferred embodiment, the transparent substrate 110 is atransparent polymeric material. Exemplary transparent polymericmaterials may include, without limitation, polyethylene terephthalate,polybutylene terephthalate, polyethylene terephthalate glycol-modified,polyethylene-2,6-naphthalate, triacetyl cellulose, liquid crystalpolymers such as thermotropic liquid crystal polyester and thermotropicliquid crystal polyester amide, acrylic resins such as polyacrylate andpolymethacrylate, olefin resins such as polyethylene and polypropylene,vinyl resins such as polyvinyl chloride, an ethylene-vinyl acetatecopolymer, and an ethylene-vinyl alcohol copolymer, imide resins such aspolyimide and polyamide-imide, and ether resins such aspolyethernitrile, polyether sulfone, polystyrene, polycarbonate,polymethylmethacrylate, polydimethylsiloxane, styrene acrylonitrile,styrene methyl methacrylate, methyl methacrylate butadiene styrene, andany combinations thereof. In a preferred embodiment, the transparentsubstrate 110 is polymethylmethacrylate (PMMA), polydimethylsiloxane(PDMS), or polycarbonate (PC).

In some embodiments, the transparent substrate 110 is an inorganictransparent material in lieu of or in addition to the transparentpolymeric material. Exemplary inorganic transparent materials that maybe used include, without limitation, an indium-doped tin oxide, afluorine-doped tin oxide, a zinc-doped tin oxide, a gallium-doped tinoxide, and combinations thereof.

The transparent electrode 114 further includes the composite layer 112,which is disposed on the transparent substrate 110. The composite layer112 includes a graphene layer 106 and a plurality of nanoparticles 102,wherein the nanoparticles 102 are embedded in the graphene layer 106 andextend through a thickness of the graphene layer 106, wherein thenanoparticles 102 are in direct contact with the transparent substrate110 and a gap 113 is present between the graphene layer 106 and thetransparent substrate 110, as shown in FIG. 1J.

The “gap” defines a distance between the graphene layer 106 and thetransparent substrate 110. In some preferred embodiments, an averagesize of the gap 113 is 0.1-10 nm, preferably 0.5-8 nm, preferably 1-6nm, preferably 2-5 nm.

In some preferred embodiments, the nanoparticles 102 are at least oneselected from the group consisting of a boron nitride nanotube, a boronnitride fullerene, a titanium oxide nanotube, and a titanium oxidefullerene. In alternative embodiments, the nanoparticles 102 may includegraphene nanoribbons, chalcogenide nanotubes, and metal chalcogenidenanotubes.

The nanoparticles 102 may have a morphology selected from the groupconsisting of a tubular morphology, a spherical morphology, anelliptical morphology, and an oblong morphology. In certain embodiments,the nanoparticles 102 may have a morphology selected from a nanosphere,a nanosheet, a nanotube, a nanofiber, a nanowire, a nanodisc, ananocube, a nanorod, a nanoring, and a nanostar.

In a preferred embodiment, the nanoparticles 102 are boron nitridenanotubes (BNNTs) with an average diameter of 3-20 nm, preferably 4-15nm, preferably 5-10 nm. The BNNTs may be single walled BNNTs orpreferably multi-walled BNNTs. The BNNTs may be present in the form offunctionalized boron nitride nanotubes, polymer wrapped boron nitridenanotubes, un-functionalized boron nitride nanotubes, and combinationsthereof. In some preferred embodiments, the nanoparticles 102 arefunctionalized boron nitride nanotubes, wherein the boron nitridenanotubes are covalently and/or non-covalently functionalized with aplurality of functional groups. Exemplary functional groups include,without limitation, alkyl groups, alcohol groups, carboxyl groups,carbonyl groups, alkoxy groups, aryl groups, aryl sulfonyl groups,polymers, sulfur groups, organic compounds, surfactants, graphenequantum dots, carbon quantum dots, inorganic quantum dots, andcombinations thereof. In more specific embodiments, the boron nitridenanotubes may be functionalized with polymers such as, withoutlimitations, poly(alkyl) oxides, poly(ethylene) oxides, poly(propylene)oxides, and combinations thereof In some embodiments, the boron nitridenanotubes may be functionalized with water soluble triblock polymers.

In certain embodiments, carbon nanomaterials may be utilized in lieu ofor in addition to the abovementioned nanoparticles 102. Exemplary carbonnanomaterials that may be utilized here may include, but are not limitedto carbon nanotube, fullerene, nanodiamond, etc. For example, in someembodiments, the nanoparticles 102 are carbon nanotubes. The carbonnanotubes may be present in the composite layer 112 in the form offunctionalized carbon nanotubes, polymer wrapped carbon nanotubes,metallic carbon nanotubes, semi-metallic carbon nanotubes, single-walledcarbon nanotubes, double-walled carbon nanotubes, multi-walled carbonnanotubes, ultra-short carbon nanotubes, and combinations thereof.

In some embodiments, the nanoparticles 102 are chalcogenide nanotubessuch as, for example, metal chalcogenide nanotubes, metalmonochalcogenide nanotubes, metal dichalcogenide nanotubes, metaltrichalcogenide nanotubes, molybdenum disulfide (MoS₂) nanotubes,molybdenum trisulfide (MoS₃) nanotubes, titanium diselenide (TiSe₂)nanotubes, molybdenum diselenide (MoSe₂) nanotubes, tungsten diselenide(WSe₂) nanotubes, tungsten disulfide (WS₂) nanotubes, niobiumtriselenide (NbSe₃) nanotubes, and combinations thereof.

In certain embodiments, the nanoparticles 102 may be present in thecomposite layer 112 as agglomerates. As used herein, the term“agglomerates” refers to clusters or clumps of the nanoparticles with anaverage diameter of no more than 3 times, preferably no more than 2times of the average diameter of the nanoparticles 102.

The term “graphene layer” as used in this disclosure refers to a layer(or a plurality of stacked layers) of graphene sheets deposited on thesurface of a CVD substrate, and the nanoparticles 102 are embedded inthe graphene layer 106 and extend through a thickness of the graphenelayer 106. The graphene layer may fill nano/micro size roughnessconcaves and/or convexes of the CVD substrate. A thickness of thegraphene layer may be different with respect to the number of graphenemonolayers (i.e. graphene sheets) that are stacked. For example, in someembodiments, the graphene layer may have 1-100 layers, preferably 2-80layers, preferably 3-50 layers, preferably 4-30 layers, preferably 5-20layers, preferably less than 15 layers of graphene monolayers that arestacked. In one embodiment, the number of layers in the graphene layeris determined by XRD. Accordingly, the thickness of the graphene layermay preferably be no more than 50 nm, preferably no more than 20 nm,preferably no more than 15 nm, preferably no more than 10 nm, preferablyin the range of 2-8 nm. In all the embodiments, the thickness of thegraphene layer 106 is always lower than the average diameter of thenanoparticles 102, and thus the nanoparticles 102 serve as pillars toraise the graphene layer 106 and form a gap 113 between the graphenelayer 106 and the transparent substrate 110, as shown in FIG. 1J. Insome embodiments, a ratio of an average thickness of the graphene layerto an average diameter of the nanoparticles is 1:1.5 to 1:10, preferably1:2 to 1:9, preferably 1:3 to 1:8.

As used herein, the term “graphene sheet” refers to a single layercrystalline material having a hexagonal honeycomb structure in whichcarbon atoms are two-dimensionally connected to each other. In someembodiments, the graphene sheet present in the composite layer 112 maybe in the form of pristine graphene, graphene oxide (GO), and/or reducedgraphene oxide (rGO). “Graphene oxide” as used here refers to anoxidized version of the graphene sheet, which includes one or moreoxygen-containing functional groups such as, e.g., carboxylate,hydroxyl, carbonyl, epoxide, etc. Said oxygen-containing functionalgroups may be present on the edge of the graphene sheet and/or on thebasal plane of the graphene sheet. “Reduced graphene oxide” as used hererefers to a kind of graphene oxide with a lower surface concentration ofthe oxygen-containing functional groups, and therefore, have a lowerinterlayer spacing compare to the graphene oxide or pristine graphene.

In one embodiment, the graphene layer 106 present in the composite layer112 may have an average bulk density in the range of 0.05 to 0.5 g/cc,preferably 0.1 to 0.45 g/cc, preferably 0.2 to 0.4 g/cc. A carboncontent of the graphene layer 106 may be at least 95 wt %, preferably atleast 98 wt %, preferably at least 99 wt %, with less than 5 wt %,preferably less than 2 wt %, preferably less than 1 wt % of impurities,wherein the weight percentile is relative to the total weight of thegraphene layer. For example, in an embodiment, the graphene layer 106may include a graphene derivative such as, for example, doped graphene,fluorographene, and/or chlorographene.

The nanoparticles 102 and the graphene layer 106 may be associated withone another through various types of interactions such as, for instance,ionic bonds, covalent bonds, non-covalent bonds, van der Waals forces,electrostatic interactions, London dispersion forces, π-π stackinginteractions, and combinations thereof.

In some preferred embodiments, the composite layer 112 further includesa dopant 108, which is dispersed in or on the graphene layer 106, asshown in FIGS. 1H, 1I, and 1J. The dopant 108 may be a p-type dopant oran n-type dopant. In terms of the present disclosure, “dopant” is atrace impurity element that is inserted into the graphene layer 106 invery low concentrations to alter electronic and/or optical properties ofthe composite layer 112. In some other embodiments, the addition of thedopant 108 to the graphene layer 106 turns the composite layer 112 intoa p-type semiconductor (positive charge carriers) or an n-typesemiconductor (negative charge carriers). The addition of the dopant 108to the graphene layer 106 may shift Fermi levels of the graphene sheets,thus changing the electronic properties of the composite layer 112. Toturn composite layer 112 into a p-type semiconductor, in someembodiment, a p-type dopant is dispersed in the graphene layer 106,wherein the p-type dopant is a boron-containing gas, preferablydiborane. In an alternative embodiment, to turn composite layer 112 intoan n-type semiconductor, an n-type dopant is dispersed in the graphenelayer 106, wherein the n-type dopant is a nitrogen-containing gas,preferably ammonia. The p-type and the n-type dopant cannot be presentin the composite layer 112 at the same time.

In one embodiment, the composite layer 112 further includes quantumdots, which may preferably be dispersed in the graphene layer 106 and/ordisposed on a surface of the graphene layer 106 and/or the nanoparticles102. As used here, the term “quantum dots” refers to semiconductingparticles having diameters in the range of 1 to 10 nm, preferably 2 to 8nm, more preferably 3 to 6 nm. The quantum dots may be added to adjustelectronic properties (e.g. band-gap energy) of the composite layer 112.The quantum dots may be one or more of core-type quantum dots,core-shell quantum dots, and alloyed quantum dots, as known to thoseskilled in the art. Exemplary quantum dots, without limitation, includechalcogenides (i.e. selenides or sulfides) of transition metals,preferably one or more of CdSe, ZnSe, CdSs/ZnS, CdS/ZnS, CdTe, PbS,InP/ZnS, PbSe, etc.

The presence of the gap 113 between the graphene layer 106 and thetransparent substrate 110 may allow electrons of graphene to freely movewithout interference from defects present on the surface of thesubstrate 110 when pulled by an electric field. In view of that, in somepreferred embodiments, an electron mobility of the composite layer 112is at least 30,000 cm²/(V·s), preferably at least 40,000 cm²/(V·s),preferably 40,000 to 80,000 cm²/(V·s), preferably 45,000 to 70,000cm²/(V·s), preferably 50,000 to 60,000 cm²/(V·s) at room temperature(i.e. a temperature of 20-30° C., preferably 22-28° C.). In view ofthat, an adverse effect of the presence of the transparent substrate 110on the electron mobility of graphene in the composite layer 112 issubstantially reduced. The term “electron mobility” as used here refersto a characteristic of a semiconductor that defines how quickly anelectron can move through the semiconductor, when pulled by an electricfield. In one embodiment, the electron mobility of the composite layer112 is calculated by measuring a drift velocity of electrons in thecomposite layer 112 when exposed to an electric field (E), as known tothose skilled in the art. In some embodiments, the composite layer 112has an ambipolar activity.

In some embodiments, a surface resistivity of the composite layer 112 ofthe present disclosure may be in the range of 10-500 Ω/square,preferably 20-400 Ω/square, preferably 30-300 Ω/square, preferably40-200 Ω/square, preferably 50-100 Ω/square.

In another preferred embodiment, the composite layer 112 has a lighttransmittance of at least 95%, preferably 96-99%, preferably 97-98%,when exposed to a light radiation with a wavelength of 300-1,000 nm,preferably 350-900 nm, preferably 400-800 nm. In view of that and thelight transmittance of the transparent substrate 110, the transparentelectrode 114 may have a light transmittance of at least 50%, preferablyat least 60%, preferably 65-95%, preferably 75-90% when exposed to alight radiation with a wavelength of 300-1,000 nm, preferably 350-900nm, preferably 400-800 nm.

In view of the aforementioned properties of the composite layer 112, thetransparent electrode 114 may be utilized in various applications due toa high electron mobility and light transmittance, and low surfaceresistivity of the composite layer 112. Exemplary applications of thetransparent electrode 114 include, without limitation, photovoltaiccells, solar cells, dye-sensitized solar cells, transistors, fieldeffect transistors, etc. In any of the abovementioned applications, thecomposite layer 112 may be mechanically flexible or rigid.

Referring to FIGS. 2A and 2B, according to a second aspect, the presentdisclosure relates to a solar cell 200 that uses the transparentelectrode 114 as a working electrode. In terms of the presentdisclosure, a solar cell is a device that collects solar radiation andcreates electrical charges to power an external electrical circuit or anelectric device. A solar cell has a transparent front side that facesthe sun during normal operation to collect solar radiation and abackside opposite the transparent front side. Solar radiation impingingon the solar cell creates electrical charges via a photovoltaic effect.

Accordingly, the solar cell 200 in accordance with the second aspectincludes the transparent electrode 114 that faces the sun to collectsolar radiation 210, a cathode 204 (or counter electrode) and anelectrolyte 206 arranged between the transparent electrode 114 and thecathode 204, wherein the transparent electrode 114 is in electriccommunication with the cathode 204 through the electrolyte 206. When thegraphene layer 106 in the transparent electrode 114 absorbs solarradiation 210, electrons in the graphene layer turn into excitedelectrons and are injected into a conduction band of the graphene layer.Said electrons may flow to the cathode 204 (or the counter electrode)through an outer circuit including a load 208. The cathode 204 (counterelectrode) is mainly used for collecting electrons and acceleratingelectron exchange rates between the electrolyte 206 and the cathode 204(or counter electrode). The high electron mobility in the compositelayer 112 of the transparent substrate 110 may increase an efficiency ofthe solar cell 200 by at least 10%, preferably at least 20%, preferably30-60%, relative to the efficiency of a conventional solar cell thatincludes a conventional working electrode in lieu of the transparentelectrode 114.

The cathode 204 may preferably be a conductive material selected fromthe group consisting of gold, platinum, silver, copper, aluminum, zinc,nickel, iron, tin, lead, and alloys or combinations thereof. In apreferred embodiment, the conductive material of the cathode 204 is anelectrochemically stable material such as, e.g., platinum, gold, carbon,and the like. In one embodiment, the cathode 204 may be a planarelectrode that is disposed parallel and across from the transparentelectrode 114, as shown in FIG. 2A. In certain embodiments, thetransparent electrode 114 is in the form of a hollow cylinder and thecathode 204 is a cylinder that is disposed inside the transparentelectrode 114 and the electrolyte 206 is arranged therebetween. Anelectrical conductivity of the cathode 204 at room temperature (i.e. atemperature of 20-30° C., preferably 22-28° C.) may be preferably withinthe range of 3.0×10⁶ to 7.0×10⁷ s/m, preferably 5.0×10⁶ to 5.0×10⁷ s/m,preferably 10⁷ to 2.0×10⁷ s/m.

The electrolyte 206 as used here refers to a non-metallic element of thesolar cell 200 that electronically/ionically connects the transparentelectrode 114 to the cathode 204. In a preferred embodiment, theelectrolyte 206 contains an aqueous sodium bicarbonate solution with aconcentration of 0.1 to 5.0 M, preferably 0.2 to 4.0 M, preferably 0.3to 3.0 M. The electrolyte 206 may be one or more of sodium chloride,sodium carbonate, potassium nitrate, calcium chloride, sodium hydroxide,potassium hydroxide, sodium acetate, and magnesium hydroxide. Theelectrolyte 206 may include at least one conductive polymer selectedfrom the group consisting of polypyrrole, polyaniline, polythiophene,poly(3,4-ethylenedioxy-thiophene), poly(3-alkylthiophenes),polyacetylene, polyphenylene vinylene, and polyphenylene sulfide. Theelectrolyte 206 may include an ionic liquid such as, for example,2-methylimidazole, 2-ethyl-4-methylimidazole,2-benzyl-4-methylimidazole, 1-(2-cyanoethyl)-2-ethyl-4-methylimidazole,1-cyanoethyl-2-methylimidazole,1-(2-cyanoethyl)2-phenyl-4,5-di-(cyanoethoxymethyl) imidazole, andcombinations thereof. In another embodiment, the electrolyte 206solution may include an organic solvent, a halogenated metal salt,and/or a halogenated nitrogen-containing heterocyclic compound. Theelectrolyte 206 may have a pH in the range of 6 to 10, preferably 7 to9. The electrolyte 206 may be present in a liquid state or in a solidstate.

Alternatively, the transparent electrode 114 may be utilized in varioustypes of solar cells or photovoltaic cells as known to those skilled inthe art including, for example, amorphous silicon solar cells, biohybridsolar cells, buried contact solar cells, cadmium telluride solar cells,concentrated PV solar cells, copper indium gallium selenide solar cells,dye-sensitized solar cells, gallium arsenide germanium solar cells,hybrid solar cells, luminescent solar concentrator cells, micromorphcells, monocrystalline solar cells, multi-junction solar cells,nanocrystal solar cells, organic solar cells, perovskite solar cells,photoelectrochemical cells, plasmonic solar cells, plastic solar cells,polycrystalline solar cells, polymer solar cells, quantum dot solarcells, solid-state solar cells, thin-film solar cells, and wafer solarcells.

Referring to FIGS. 2C and 2D, one aspect of the present disclosurerelates to a dye-sensitized solar cell 202 that uses the transparentelectrode 114 as a working electrode.

A dye-sensitized solar cell, as used here, refers to an electricaldevice that converts the energy of light directly into electricity (i.e.a photoelectric conversion mechanism) by a photovoltaic effect throughthe use of dye particles. Accordingly, the dye-sensitized solar cell 202includes the transparent electrode 114 that faces the sun to collectsolar radiation 210, a cathode 204, an electrolyte 206 arranged betweenthe transparent electrode 114 and the cathode 204, optionally aplurality of semiconducting particles 212 disposed in the electrolyte206, and dye particles 214 arranged on the semiconducting particles 212.Accordingly, the transparent electrode 114 is in electric communicationwith the cathode 204 through the electrolyte 206. When thedye-sensitized solar cell 202 is exposed to solar radiation 210, the dyeparticles 214 absorb energy from the solar radiation 210, and electronsin the dye particles 214 turn into excited electrons and injected into aconduction band of the semiconducting particles 212, thereby causing thedye particles 214 to oxidize due to loss of electrons. In theembodiments where semiconducting particles 212 is not present, theexcited electrons are injected into a conduction band of the compositelayer 112, which is an n-type semiconductor. The electrons that areinjected into the conduction band of the semiconducting particles 212,may accumulate on the transparent electrode 114, and may further flow tothe cathode 204 through an outer circuit including a load 208. On theother hand, oxidized dye particles accept electrons from the electrolyte206 and return to the ground state. The electron donor in theelectrolyte 206 diffuses to the cathode 204 and accepts an electron andis reduced at the cathode 204. The high electron mobility in thecomposite layer 112 of the transparent substrate 110 may increase anefficiency of the dye-sensitized solar cell 202 by at least 10%,preferably at least 20%, preferably 30-60%, relative to the efficiencyof a conventional dye-sensitized solar cell that includes a conventionalworking electrode in lieu of the transparent electrode 114.

The semiconducting particles 212, when present, may preferably be n-typesemiconductors that are multi-component nanocomposite particlespreferably with a perovskite structure. The multi-componentnanocomposite particles may be an oxide of a metal selected from thegroup consisting of Ti, Zr, Sr, Zn, In, Yr, La, V, Mo, W, Sn, Nb, Mg,Al, Y, Sc, Sm, Ga, and TiSr. For example, the multi-componentnanocomposite particles may be TiO₂, SnO₂, ZnO, WO₃, Nb₂O₅, TiSrO₃, or amixture thereof. The semiconducting particles 212, when present, maypreferably be arranged between the transparent electrode 114 and thecathode 204 preferably in direct contact with the composite layer 112 ofthe transparent electrode 114, as shown in FIG. 2D.

The dye particles 214 may preferably be utilized to enhance theabsorption of light photons onto the transparent electrode 114 and/orthe semiconducting particles 212, when present. The dye particles 214may include an azin dye, an azo dye, a diarylmethane dye, a fluorescentdye, a food coloring, a fuel dye, an ikat dye, an indigo structured dye,an indophenol dye, a perylene dye, a phenol dye, a quinoline dye, arhodamine dye, a solvent dye, a staining dye, a thiazine dye, a thiazoledye, a triarylmethane dye, a vat dye, a violanthrone dye, etc. Forexample, in one embodiment, the dye particles 214 include is a thiazinedye, in particular, methylthioninium chloride (methylene blue). Incertain embodiments, the dye particles 214 may include a rutheniumcomplex, for example, a xanthine-based pigment (e.g. rhodamine B, rosebengal, eosin, or erythrocin), a cyanine-based pigment (e.g.quinocyanine, merocyanine, naphthalocyanine, or cryptocyanine), a basicdye (e.g. phenosafranine, Capri blue, thiocine, or methyleneblue), aphosphirine-based compound (e.g. chloropyl, zinc phosphirine, ormagnesium phosphirine), other azo pigments, a complex compound (e.g. aphthalocyanine compound, or Ru trisbipyridyl), an anthraquinone-basedpigment, or a polycyclic quinine-based pigment.

According to a third aspect, the present disclosure relates to a methodof fabricating the transparent electrode 114.

The method involves depositing the nanoparticles 102 on a surface of aCVD substrate 104. Depositing the nanoparticles 102 on the surface ofthe CVD substrate 104 may be carried out using methods known to thoseskilled in the art, for example, dispersion, drop-casting, sputtering,physical application, spraying, vapor-coating, chemical vapor deposition(CVD), and combinations thereof.

In a preferred embodiment, the nanoparticles 102 are dispersed in anorganic solvent, and further sonicated, preferably ultra-sonicated toform a deposition precursor. The deposition precursor is then sprayed onthe CVD substrate 104. The deposition precursor may alternatively bedeposited by any methods as known to those skilled in the art including,without limitation, brushing, dipping, drop-coating, pouring, orspin-coating. In some preferred embodiments, an automatic spray gun,e.g. a spray atomizer, is utilized to uniformly spray the depositionprecursor onto the CVD substrate 104. The organic solvent may be analcohol-based organic solvent, an ether-based organic solvent, aketone-based organic solvent, an ester-based organic solvent and/or anorganic acid-based organic solvent.

Once the deposition precursor is dispersed, the CVD substrate 104 isheated to a temperature above a boiling temperature of the organicsolvent to evaporate the organic solvent to form a layer of thenanoparticles 102 on the CVD substrate 104.

In some embodiments, the nanoparticles 102 are deposited on the surfaceof the CVD substrate 104 in an inert environment, which is provided by astream of an inert gas such as, without limitation, N₂, Ar, He, andcombinations thereof.

In one embodiment, the CVD substrate 104 is any kind of substrates thatcommonly utilized in chemical vapor deposition, as known to thoseskilled in the art. For example, in a preferred embodiment, the CVDsubstrate 104 is a copper substrate. The CVD substrate 104 mayalternatively be a cobalt substrate or a nickel substrate. In certainembodiments, the CVD substrate 104 may include at least one elementselected from the group consisting of Cu, Ni, Co, Fe, Pt, Au, Al, Cr,Mg, Mn, Mo, Rh, Si, Ta, Ti, W, U, V, and Zr. The CVD substrate 104 mayhave various sizes and shapes. For instance, in some embodiments, theCVD substrate 104 may have surface sizes that range from about 10 nm toabout 10 cm in length or width. In some embodiments, the CVD substrate104 may be pre-patterned. In some embodiments, the CVD substrate 104 maybe in the shapes of squares, rectangles, triangles, or other similarshapes. In some embodiments, the CVD substrate 104 may be planar, rolledor coiled.

In some embodiments, the methods of the present disclosure also includea step of cleaning the CVD substrate 104 prior to depositing thenanoparticles 102 thereon. For example, in one embodiment, the cleaninginvolves electrochemical-polishing the CVD substrate by applying avoltage to the CVD substrate and polishing the CVD substrate for acertain amount of time. For instance, in more specific embodiments,electrochemical polishing may include applying a voltage of about 0.5 Vor higher to the surface of the CVD substrate, and polishing the surfaceof the CVD substrate for about 10 seconds or longer. In someembodiments, the cleaning of CVD substrate may occur by mechanicalpolishing, acid cleaning, and/or high temperature annealing underreductive or inert atmospheres.

In certain embodiments, the deposition precursor may subsequently bedispersed on the CVD substrate 104 for at least one cycle afterevaporation of the organic solvent, to form at least one layer ofnanoparticles on the CVD substrate. Alternative to the abovementioneddeposition method, the nanoparticles may be deposited on the surface ofthe CVD substrate by a method such as screen printing, doctor bladecoating, gravure coating, silk screening, painting, slit die coating,roll coating, decalomania coating, and combinations thereof. Selectionof a suitable method of depositing the nanoparticles on the surface ofthe CVD substrate may depend on the type of the nanoparticles and aviscosity of the deposition precursor.

Once the nanoparticles 102 are deposited on the surface of the CVDsubstrate 104, in a next step, graphene is deposited on the CVDsubstrate 104, for example, using chemical vapor deposition (CVD) asknown to those skilled in the art. Accordingly, the CVD substrate withthe nanoparticles are first heated to a temperature of at least 900° C.,preferably 1050-1200° C., preferably around 1000° C. in asub-atmospheric pressure, e.g. a pressure of less than 0.8 atm,preferably less than 0.2 atm, preferably less than 0.1 atm. The CVDsubstrate with the nanoparticles may be heated in a furnace or an oven,preferably a vacuum oven. In some embodiments, CVD substrate and thenanoparticles deposited thereon may be heated with induction heatingusing various energy sources such as RF radiating energy, for example,lasers, infrared rays, microwaves, high energy X-rays, and combinationsthereof.

In some embodiments, the CVD substrate and the nanoparticles are heatedin an inert environment, which is preferably provided by a stream of aninert gas such as, without limitation, N₂, Ar, He, and combinationsthereof. The inert gas may be applied to at a flow rate of 5-500 sccm,preferably 50-100 sccm, and a pressure of 1-20 Torr, preferably 2-10Torr.

Once the CVD substrate and the nanoparticles are equilibrated at thetemperature of at least 900° C., preferably 1050-1200° C., preferablyaround 1000° C. in the sub-atmospheric pressure, a gaseous mixturecomprising methane and hydrogen is contacted with the nanoparticles andthe CVD substrate, wherein hydrogen catalyzes a reaction between methaneand the CVD substrate, thus causing carbon atoms to be deposited on thesurface of the CVD substrate through a chemical adsorption process.Accordingly, the carbon atoms are deposited on the CVD substrate in theform of graphene sheets, thus forming the graphene layer 106. Due to theexistence of the nanoparticles on the surface of the CVD substrate, thegraphene grows on those surface areas that are not occupied by thenanoparticles 102, as shown in FIGS. 1D and 1E. As a result, thegraphene layer 106 surrounds the nanoparticles 102 such that thenanoparticles are embedded in the graphene layer and extend through athickness of the graphene layer. A growth rate of the graphene layersmay preferably be controlled by controlling a temperature and a flowrate of the gaseous mixture, such that the ratio of an average thicknessof the graphene layer 106 to an average diameter of the nanoparticles102 ranges from 1:1.5 to 1:10, preferably 1:2 to 1:9, preferably 1:3 to1:8. The gaseous mixture is contacted with the nanoparticles 102 and theCVD substrate 104 for a period of 1-10 minutes, preferably 2-8 minutes,preferably 4-6 minutes to form the graphene layer. The graphene layermay preferably include multiple layers of graphene sheet that arestacked together and are weakly bonded by Van der Waals and/or π-πstacking interactions. The graphene layer may preferably includecrystalline and polycrystalline layers, although in certain embodimentsformation of amorphous carbon is also envisaged. The graphene layer mayinclude from 1 to about 100 monolayers, preferably 1 to about 50monolayers, from 1 to about 20 monolayers, preferably from 1 to about 10monolayers, preferably from 1 to about 5 monolayers of graphene sheets.The term “monolayer” as used herein in connection with graphene refersto a single atom-thick layer of graphene or “graphene sheet.” In termsof the present disclosure, the term “monolayer” and the term “graphenesheet” are identical and may be used interchangeably.

In certain embodiments, the gaseous mixture further includes a dopant108, which may be a p-type dopant or an n-type dopant. Accordingly, atrace amount of the dopant 108 may be inserted into the composite layer112 during formation of the graphene layer. In view of that, a p-typesemiconductor (positive charge carriers) or an n-type semiconductor(negative charge carriers) may be fabricated by adding a p-type dopantor an n-type dopant in a composition of the gaseous mixture. Forexample, in one embodiment, the gaseous mixture includes diborane, andthe composite layer 112 turns into a p-type semiconductor. In analternative embodiment, the gaseous mixture includes ammonia, and thecomposite layer 112 turns into an n-type semiconductor.

After contacting the gaseous mixture and formation of the compositelayer, the CVD substrate 104 and the composite layer 112 formed thereonis cooled preferably at a cooling rate of 10-30° C/min, preferably15-25° C/min. Cooling the CVD substrate 104 and the composite layer 112at the abovementioned cooling rates preferably prevent formation of bulkgraphite.

Referring to FIG. 1J step (1), the method further involvesdisposing/placing the transparent substrate 110 on the CVD substrate 104and the composite layer 112. Accordingly, the transparent substrate 110,which may preferably be in the form of a pellet, is placed on thecomposite layer 112, wherein the transparent substrate 110 is in directcontact with the nanoparticles 102 and a gap 113 is present between thetransparent substrate 110 and the graphene layer 106.

In some preferred embodiments, the transparent substrate 110 in the formof pellets, are pre-treated to form oxygen functional groups, e.g.,hydroxyl, carbonyl, carboxyl, aldehyde, etc., on the surface of thetransparent substrate 110 before disposing/placing on the compositelayer 112. Such pre-treatment steps may involve one or more of plasmatreatment, acid-treatment, corona treatment, and flame treatment. Thepresence of the aforementioned oxygen functional groups on a surface ofthe transparent substrate may facilitate the step of disposing/placingthe transparent substrate on the composite layer, and may prolong a lifetime of the transparent electrode.

To carry out a plasma treatment, the pellets of the transparentsubstrate 110 may preferably be placed in a plasma chamber, and a plasmagas with a sub-atmospheric pressure (i.e. a pressure of 0.9 atm tovacuum, or preferably 0.6 atm to 0.1 atm, or preferably 0.4 atm to 0.2atm) may be introduced into the plasma chamber. The plasma gas maypreferably be oxygen, ammonia, argon, and/or nitrogen, although an inertgas such as helium may also be used. As used herein, the term “plasmagas” may refer to a gaseous matter that contains ions and electrons. Oneway to form the plasma gas may be through electrically charging an inertgas. Plasma treating the transparent substrate may form oxygenfunctional groups, e.g., hydroxyl, carbonyl, carboxyl, aldehyde, etc. ona portion of a surface of the transparent substrate by causing ions andelectrons present in the plasma gas to interact with the surface of thetransparent substrate. The transparent substrate may be plasma treatedfor 1 to 5 minutes, preferably 2 to 4 minutes under the plasma gas.After plasma treating, the transparent substrate may be washed with anorganic solvent, e.g. acetone, an alcohol (e.g., ethanol), toluene,water, etc.

The transparent substrate may have chemically inert and nonporoussurfaces with a low surface tension, thus being non-receptive to createbonds with the nanoparticles of the composite layer. Accordingly, insome embodiments, the transparent substrate may be corona treated. Inview of that, the pellets of the transparent substrate may be subjectedto a plasma that may be generated in air at an atmospheric pressure(i.e. about 1 atm) by applying an AC voltage to two electrodes of acorona treating device. The plasma may contain a number of energeticions and electrons that may form oxygen functional groups, e.g.,hydroxyl, carbonyl, carboxyl, aldehyde, etc. on a portion of the surfaceof the transparent substrate. Corona treating the transparent substratemay enhance a surface energy of the transparent substrate. Coronatreating may further enhance a surface roughness of the transparentsubstrate by oxidizing a portion of the surface of the transparentsubstrate, thus improving adhesion bonds between the transparentsubstrate and the nanoparticles of the composite layer.

In some embodiments, the transparent substrate may be acid-treated withan acid solution in order to remove impurities from the surfaces of thetransparent substrate. Acid-treating the transparent substrate mayconsiderably affect a surface morphology and/or a surface chemistry ofthe transparent substrate, for example, by creating oxygen functionalgroups, e.g., hydroxyl, carbonyl, carboxyl, aldehyde, etc. on thesurface of the transparent substrate. Exemplary acid solutions that maybe utilized here include, without limitation, an iodic acid solution, apermanganic acid solution, a nitric acid solution, and/or a chromic acidsolution. Preferably, the concentration of the acid solutions may beless than 2.0 M, preferably less than 1.5 M.

Additionally, the transparent substrate may be flame treated preferablyto break a portion of molecular chains on a surface of the transparentsubstrate, thus creating polar functional groups on the surface of thetransparent substrate. Accordingly, the transparent substrate may besubjected to a blue flame, e.g. an oxygen-containing propane flame or anoxygen-containing acetylene flame for a few seconds, preferably no morethan 20 seconds, preferably no more than 10 seconds. Additionally, flametreating the transparent substrate may advantageously burn off dustparticles, and impurities that may be present on the surface of thetransparent substrate and adversely affect bonding strength between thetransparent substrate and the nanoparticles.

In certain embodiments, disposing/placing the transparent substrate 110on the composite layer 112 may be carried out by spin/spray coating apolymeric solution onto the composite layer 112, followed by a dryingstep. The polymeric solution may be formed by dissolving the transparentpolymeric material, as mentioned previously, in an organic solvent. In apreferred embodiment, the polymeric solution is formed by dissolvingpolymethylmethacrylate and/or polydimethylsiloxane in the organicsolvent. Examples of the organic solvent that may be utilized hereinclude, without limitation, an alcohol-based organic solvent, anether-based organic solvent, a ketone-based organic solvent, anester-based organic solvent and/or an organic acid-based organicsolvent. Examples of alcohol-based organic solvents may includemonovalent alcohols and polyvalent alcohols, which may be used alone orin a combination thereof. Examples of monovalent alcohols may includepropanol, pentaol, hexanol, heptanol, and octanol, and examples ofpolyvalent alcohols may include propylene glycol, diethylene glycol,dipropylene glycol, triethylene glycol, tripropylene glycol, octyleneglycol, tetraethylene glycol, neopentyl glycol, 1,2-butandiol,1,3-butandiol, 1,4-butandiol, 2,3-butandiol, 1,2-dimethyl-2,2-butandiol,and 1,3-dimethyl-2,2-butandiol.

Referring to FIG. 1J step (2), after disposing/placing the transparentsubstrate 110 on the composite layer 112, the CVD substrate 104 isremoved, preferably with an etchant, leaving behind the transparentelectrode 114, wherein the nanoparticles 102 are in direct contact withthe transparent substrate 110 and a gap 113 is present between thegraphene layer 106 and the transparent substrate 110. In one embodiment,the etchant is preferably a ferric chloride solution. The ferricchloride solution may be 10-50% by weight, preferably 30-45% by weightof ferric chloride. The CVD substrate may preferably be etched in acommercially available etching machine. Alternative etchants may also beutilized to remove the CVD substrate such as, for example, ammoniumhydroxide, cupric chloride, chromium trioxide, and sulfuric acid. In oneembodiment, the etchant may be an aqueous solution of (NH₄)₂S₂O₈ with 1%by volume of butanol.

Referring to FIG. 1J step (3) and (4), in some embodiments, thetransparent substrate 110 is removed with the organic solvent, thusleaving behind the composite layer 112. Accordingly, the composite layer112 may be subjected to further processing steps to be utilized invarious applications, e.g. solar cells, transistors, etc. For example,in some embodiments, the method further involves the steps oftransferring the composite layer 112 onto a pre-treated substrate 115(e.g. a plasma treated substrate) such as, without limitation, SiO₂/Siwafers, Al₂O₃, Si₃N₄, SiC, and combinations thereof. In one embodiment,transferring the composite layer 112 involves dipping the compositelayer 112 into a solution. In another embodiment, transferring thecomposite layer 112 involves manually placing the composite layer 112onto the pre-treated substrate 115. Additional methods by which toremove the composite layer 112 from the transparent substrate 110 and/orto transfer the composite layer to another substrate is also beenvisioned.

Thus, the foregoing discussion discloses and describes merely exemplaryembodiments of the present invention. As will be understood by thoseskilled in the art, the present invention may be embodied in otherspecific forms without departing from the spirit or essentialcharacteristics thereof. Accordingly, the disclosure of the presentinvention is intended to be illustrative, but not limiting of the scopeof the invention, as well as other claims. The disclosure, including anyreadily discernible variants of the teachings herein, defines, in part,the scope of the foregoing claim terminology such that no inventivesubject matter is dedicated to the public.

1. A transparent electrode comprising: a transparent substrate; and a composite layer disposed on the transparent substrate, wherein the composite layer comprises a graphene layer and a plurality of nanoparticles, wherein the nanoparticles are embedded in the graphene layer and extend through a thickness of the graphene layer, wherein a ratio of the thickness of the graphene layer to an average diameter of the plurality of nanoparticles is 1:1.5 to 1:10, wherein at least a portion of the plurality of nanoparticles are in direct contact with the transparent substrate and a gap is present between the graphene layer and the transparent substrate, wherein the transparent substrate consists of one or more selected from the group consisting of quartz, glass, polymethylmethacrylate, polydimethylsiloxane, polymethacrylate, and polycarbonate.
 2. (canceled)
 3. The transparent electrode of claim 1, wherein the plurality of nanoparticles are at least one selected from the group consisting of a boron nitride nanotube, a boron nitride fullerene, a titanium oxide nanotube, and a titanium oxide fullerene.
 4. The transparent electrode of claim 1, wherein the plurality of nanoparticles are boron nitride nanotubes with an average diameter of 3-10 nm.
 5. The transparent electrode of claim 1, wherein a morphology of the plurality of nanoparticles is at least one selected from the group consisting of a tubular morphology, a spherical morphology, an elliptical morphology, and an oblong morphology.
 6. The transparent electrode of claim 1, wherein the gap defines a distance between the graphene layer and the transparent substrate, and wherein an average size of the gap is 0.1-10 nm.
 7. The transparent electrode of claim 1, wherein the composite layer further comprises a p-type dopant, which is dispersed in the graphene layer.
 8. The transparent electrode of claim 7, wherein the p-type dopant is diborane.
 9. The transparent electrode of claim 1, wherein the composite layer further comprises an n-type dopant, which is dispersed in the graphene layer.
 10. The transparent electrode of claim 9, wherein the n-type dopant is ammonia.
 11. The transparent electrode of claim 1, wherein the composite layer further comprises quantum dots.
 12. (canceled)
 13. The transparent electrode of claim 1, wherein the composite layer has an electron mobility of at least 30,000 cm²/(V·s) at a temperature of 20-30° C.
 14. The transparent electrode of claim 1, wherein the composite layer has a light transmittance of at least 95% when exposed to a light radiation with a wavelength of 300 to 1,000 nm.
 15. A solar cell comprising: the transparent electrode of claim 1; a cathode; and an electrolyte arranged between the transparent electrode and the cathode, wherein the transparent electrode is in electric communication with the cathode through the electrolyte. 16-20. (canceled) 